Single Wall Nanotubes (SWNTs)
There has been an intense interest in carbon nanotubes (CNTs) since their discovery by Iijima in 1991, in large part because they possess unique structural and electronic properties. Single wall carbon nanotubes (SWNTs) are the fundamental form of carbon nanotubes with unique electronic properties that emerge due to their one dimensionality. An SWNT comprises a single hexagonal layer of carbon atoms (a graphene sheet) that has been rolled up to form a seamless cylinder. Three types of SWNTs with differing chirality are expected to open new frontiers with applications ranging from new materials, to electronics and molecular scale sensing. Several processes for large scale synthesis/manufacture of SWNTs are also being developed by various research groups around the globe. These include Laser ablation (PLV) reported by Smalley's and Eklund's group (Guo et al., Chem Phys Lett., 243 (1995) 49; Eklund et al., Nano Letters (2002), 2(6), 561-566), arc discharge by Journet and coworkers (Journet et al., Nature (London) (1997), 388 (6644), 756-758) and chemical vapor deposition (CVD) method by several different groups (Nikolav et al., Chem. Phys. Lett., 313 (1999) 91; Wang et al., Chem. Phys. Lett., 364 (2002) 568-572; Kato et al., Thin Solid Films, 457 (2004) 2-6; Lyu et al., J. Phys. Chem. B, (2004), 108, 1613-1616). CVD methods include high pressure and catalytic CVD. SWNTs produced from different methods show slight variations in their electronic properties, and in size distribution (Kuzmany et al., Synthetic Metals, 141 (2004) 113-122). (All references cited in this paragraph are herein incorporated by reference in their entirety).
Functionalization of SWNTs has been of much interest to the scientific community because it enhances applicability. For example, insoluble SWNTs can be rendered soluble, which will lead to easy processibility, as working with a suspension is always a challenge. Functionalization may also lead to more efficient purification/separation techniques, such as, those based on chirality, or, the separation of metallic SWNTs from semi-conducting ones. More importantly, functionalization leads to the development of new classes of material with specificity for different physical and chemical properties.
SWNTs have no functional groups and are consequently quite inert. Limited reactivity arises due to the curvature induced stress from the non-planer sp2 carbons and the misaligned n orbitals. While there is a wealth of literature on the derivatization of the SWNTs, the two most general approaches appear to be 1,3-dipolar cycloaddition, and oxidation of some of the atoms at the tube ends or on the tube wall, and then substitution of the functionality thus formed (—F, —OH, —COOH). At this point, a variety of synthetic organic reactions can be carried out. An example of the former approach is a reaction with azomethine. The latter approach, on the other hand, requires a more aggressive oxidation, such as, refluxing with HNO3, ozonation, or reaction with solid KOH.
Much of the effort so far has involved the use of conventional techniques such as refluxing and sonication. For different functionalization purposes, carbon nanotubes are usually treated in different solvents by refluxing, or heating and stirring. Many of these reactions need to be carried out over a long period of time. For example, for generating carboxyl groups, carbon nanotubes are often refluxed in concentrated HNO3 for tens of hours; thereafter, several days are required for refluxing (or heating) for further functionalization in processes such as acyl chlorination and amidation, diimide-activated amidation, or 1,3-dipolar cycloaddition.
Acid treatment has been the most commonly used functionalization approach. It leads to debundling of the nanotubes, and is the first step towards amidation, esterification and other applications. Conventional acid treatment is a long process, however, as it requires several hours to several days depending upon the requirements of the final product. Further, most of the reported methods also involve multiple steps, and only a limited solubility has been achieved (order of few milligrams per liter) (see for example, Loupy A. Solvent-free microwave organic synthesis as an efficient procedure for green chemistry, C. R. Chimie (2004) 7(2): 103-112; Lewis et al., Accelerated imidization reactions using microwave Radiation, J. Polym. Sci. A (1992) 30:1647-1653). The development of a fast, efficient and controllable technique for SWNT functionalization will dramatically speed-up their real world applications.
A key issue in functionalization of SWNTs has been the desire to increase their solubility in both water and organic solvents. One disadvantage of many nanomaterials is their limited solubility in common solvents. Solubility of nanomaterials, specifically carbon nanotubes, in water would allow chemical derivatization and manipulation of the nanotubes to be facilitated simply and less expensively. Due to the tremendous benefits that soluble nanomaterials would create, considerable efforts have therefore been made in the past to make carbon nanotubes soluble in water and in organic solvents, but to date have been met with only limited success. Moreover, the solubilities achieved are mostly due to water-soluble macromolecules attached to the nanotubes, rather than the development of a soluble nanomaterial.
Microwaves
Microwaves are electromagnetic radiation in the 0.3-300 GHz frequency range (corresponding to 0.1-100 cm wavelength). To avoid interference with communication networks, all microwave heaters (domestic or scientific) are designed to work at either 2.45 GHz or 0.9 GHz, of which, the former is more prevalent. According to Planck's law, the energy at this wavelength is 0.3 cal/mol, and is therefore insufficient for molecular excitation, thus most of the energy is used in substrate heat-up. The mechanism of microwave heating is different from that of conventional heating, where heat is transferred by conduction, convection or radiation. In microwave heating, electromagnetic energy is transformed into heat through ionic conduction and the friction due to rapid reorientation of the dipoles under microwave radiation. The larger the dipole moment of a molecule, the more vigorous is the oscillation in the microwave field, consequently more heating. This type of heating is fast, has no inertia, and is in-situ without heating the surroundings.
Chemistry under microwave radiation is known to be quite different, fast and efficient (Gedye et al., Tetrahedron Lett., 27 (1986) 279; Giguere et al., Tetrahedron Lett., 27 (1986) 4945; Loupy et al., Chimie, 7 (2004) 103-112). It also reduces the need for solvents, thus it is eco-friendly. It has been exploited in a variety of organics synthesis including hetero cyclic, organometallic, and combinatorial chemistry. Some of the reported advantages are rapid reactions under controlled temperature and pressure (especially in a closed system), higher purity products achieved due to short residence times at higher temperatures, and better yields at even very short residence times. Another important factor is that during dipolar polarization under microwave radiation, the activation parameters are modified. For example, it has been reported by Lewis that during imidization of polyamic acid, the activation energy reduced from 105 to 57 KJ/mol. (Lewis et al., J. Polym. Sci., 30A (1992) 1647) (All referenced cited in this paragraph are herein incorporated by reference in their entirety)
Composites
The mechanical properties of single wall carbon nanotubes (SWNTs) such as their stiffness, elasticity and high Young's modulus, make them ideal candidates for structural reinforcements in the fabrication of high strength, light weight, and high performance composites. Considerable investigations have been conducted on the SWNT based composites by both theoretical and experimental means. These prior art approaches involve, among other things, dispersion, melt mixing, milling, covalent grafting or in-situ growing SWNTs in different polymer or ceramic matrix to achieve the certain composite. The results, however, conflict among studies wherein some studies revealed that the introduction of SWNTs in polymer clearly enhances both the physical and mechanical properties while others showed that the carbon nanotube contributed no mechanical improvement to the composites.
The ineffective utilization of nanotubes as reinforcement in composites is normally suffered from two factors, non-uniform dispersion of SWNTs in matrix and poor interfacial bonding between them. The latter one consequently will result in low efficiency of load transfer across the nanotube/matrix interface, and the pull-out of carbon nanotubes from the matrix can be observed when the composites are under extension.
The excellent mechanical, thermal and electrical properties of carbon nanotubes and SWNTs would be significantly enhanced by the development of nanocomposites containing ceramic, polymer and metal incorporated into carbon nanotubes. Desirable properties for ceramic, polymer and metal composites include mechanical toughness, wear resistance, and the reduction in crack growth coupled with improved thermal conductivity, resistance to thermal shock and increased electrical conductance. For example, the ceramics are inherently brittle and the incorporation of SWNTs is known to have improved toughness by as much as 24% (Kamalakaran et al., Microstructural characterization of C-SiC-carbon nanotube composite flakes, Carbon (2004) 42(1): 1-4).
Significant efforts have gone into theoretical and experimental investigations of carbon nanotube-based composites, but challenges in fabrication, particularly for ceramics and metals have not been overcome. Fabrication methods such as, hot pressing, sintering, milling, covalent grafting and in-situ catalytic growth in ceramic and polymer matrices via chemical vapor deposition (CVD), have been used. These methods may be classified as those where the carbon nanotubes and the ceramic were physically mixed and then bonded by heat-treatment, or those where the nanotubes were grown in a ceramic matrix via CVD. This typically generates a mixture of single and multiwall nanotubes along with amorphous carbon. As an example of the former approach, Al2O3/nanotube composites were prepared by ball milling a methanol suspension of the ceramic and nanotubes for 24 hours (Wang et al., Contact-damage-resistant ceramic/single-wall carbon nanotubes and ceramic/graphic composites, Nature Materials (2004) 3: 539-544). An example of the latter is the synthesis from a slurry of SiC and ferrocene in xylene, which was sprayed into a reactor at 1000° C. under argon (Kamalakaran et al.). The observed ineffective utilization of carbon nanotubes as the reinforcing material in many of these composites has been attributed to the non-uniform dispersion of carbon nanotubes, and the poor interfacial adhesion to the matrix. For example, the latter results in ineffective load transfer across the nanotube/matrix interface, and the “pullout” of carbon nanotubes has been observed when the composite is under strain. An important issue has been the high temperature and reactivity of some of the current methodologies, which can destroy and/or damage the carbon nanotubes.
In summary, functionalization of carbon nanotubes by known conventional methods is a tedious and time-consuming procedure. Consequently, there is a need to develop techniques for fast functionalization and solubilization of SWNTs, as well as composites thereof.